Many varieties of trunked two-way radio communications systems are known in the art. FIG. 1 is a block diagram illustrating both a typical conventional radio system 101 and a trunked radio system 103. In the conventional radio system 101, a plurality of subscriber units are formed into talkgroups. Each talkgroup uses separate channels for communication. Thus, each talkgroup is served by one channel. In contrast, the trunked radio system 103 and its subscriber units use a pool of channels for virtually an unlimited number of talkgroups. Thus, all talkgroups are served by all channels. The trunked radio system 103 works to take advantage of the probability that not all talkgroups need a channel for communication at the same time. Estimates are made about how much load a typical user presents to the system in terms of calls per hour and duration of each call. For a traffic load, fewer channels are required since all talkgroups are served by all channels. Combining this with the number of users on the system, and the acceptable quality of service (QoS), determines how many trunked channels are required to satisfactorily serve the number of users. With a given number of channels, a much greater number of talkgroups can be accommodated as compared with conventional radio systems. Hence, a primary purpose of a trunked radio system is the efficient utilization of channels allowing for more users to carry many conversations over a fewer number of distinct channels.
As seen in FIG. 2, a trunked radio system can be either a centralized trunked radio system 201 or a decentralized trunked radio system 203. A centralized trunked radio system 201 uses a dedicated or exclusive channel, which is often referred to as a control channel, for communication between subscriber units and a central controller 205. Other terms that sometimes refer to the central controller 205 include trunking controller, site controller, resource allocator, channel allocator, controller, and other like terms. The subscriber units constantly monitor the control channel for channel assignment instructions. In order to start a group call (i.e., a one-to-many call), a subscriber unit requests that a channel is allocated for its use, and the central controller 205 transmits instructions telling the subscriber units in the group to switch to a traffic channel assigned for that call. A similar process is followed when a subscriber unit starts an individual call (i.e., a one-to-one call).
A decentralized trunked radio system 203, however, does not require the use of an exclusive channel. The intelligence or control function for assignment of a channel to a call remains in the subscriber units. Thus, the decentralized trunked radio system 203 can co-exist with conventional users on the same channels without the use of the control channel. When a call is initiated by a subscriber unit, the channel assignment is determined by the logic in subscriber units, not by a controller. In operation, a subscriber unit scans the channels, finds an idle channel and starts a call on the idle channel. The disadvantage of the decentralized trunked radio system 203 is that the scan to find an idle channel significantly increases the access time, which often provides for unacceptably high latency delays during call set up.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present disclosure.